Semitransparent Perovskite Solar Cells with Ultrathin Protective Buffer Layers

Semitransparent perovskite solar cells (ST-PSCs) are increasingly important in a range of applications, including top cells in tandem devices and see-through photovoltaics. Transparent conductive oxides (TCOs) are commonly used as transparent electrodes, with sputtering being the preferred deposition method. However, this process can damage exposed layers, affecting the electrical performance of the devices. In this study, an indium tin oxide (ITO) deposition process that effectively suppresses sputtering damage was developed using a transition metal oxides (TMOs)-based buffer layer. An ultrathin (<10 nm) layer of evaporated vanadium oxide or molybdenum oxide was found to be effective in protecting against sputtering damage in ST-PSCs for tandem applications, as well as in thin perovskite-based devices for building-integrated photovoltaics. The identification of minimal parasitic absorption, the high work function and the analysis of oxygen vacancies denoted that the TMO layers are suitable for use in ST-PSCs. The highest fill factor (FF) achieved was 76%, and the efficiency (16.4%) was reduced by less than 10% when compared with the efficiency of gold-based PSCs. Moreover, up-scaling to 1 cm2-large area ST-PSCs with the buffer layer was successfully demonstrated with an FF of ∼70% and an efficiency of 15.7%. Comparing the two TMOs, the ST-PSC with an ultrathin V2Ox layer was slightly less efficient than that with MoOx, but its superior transmittance in the near infrared and greater light-soaking stability (a T80 of 600 h for V2Ox compared to a T80 of 12 h for MoOx) make V2Ox a promising buffer layer for preventing ITO sputtering damage in ST-PSCs.

The surface coverage of the V2Ox films was examined through EDX elemental maps on glass/ITO/V2Ox samples with two different thicknesses (2.5 nm, 10 nm) of the TMO layer.The analysis showed a similar surface coverage percentage of the film, from 53% of 2.5 nm-thick V2Ox (Fig. S7 -B, D) to 58% of 10 nm-thick V2Ox.Moreover, AFM measurement disclosed a quite similar morphology between 10 nm-and 2.5 nm-thick V2Ox layers, as shown in Fig. S7 -A, C. In the first case, a continuous film with a surface rms roughness, σRMS, of 3.7 nm was observed, whereas for 2.5 nm-thick V2Ox we observed a slightly more uneven structure, even though σRMS is 3.1 nm.However, a more irregular surface is not detrimental to the devices as regards the sputtering damage.Due to the presence of the heavy metal, the V2Ox is still maintaining the intrinsic property of acting as a shield against ions (such as Ar + , Ar, O − and O2 − ) and X-rays present in the plasma during the sputtering process.[1,2] Nevertheless, the possible presence of cavities can be the cause of the observed VOC reduction for thin layers since the PTAA/ITO interface can play a role in the energy band alignment.
In the case of MoOx, a higher surface coverage of ~90% was detected with EDX analyses in 7.5 nm-and 10 nm-thick layers.The surface roughness of the films is approximately 4.5 nm in both cases, meaning that a continuous layer is deposited.S3: Spectral fitting parameters of Mo 3d and Ta 4d species.
The range of BEs between 420 and 390 eV was also analyzed (Figure S8) and Mo 3p core levels were identified [3,4].The peak energies are localized at 398.0 eV and 415.6 eV for the spin-orbit doublet 3p3/2 and 3p1/2, respectively.Other peaks in this BE interval were detected, which are likely to be due to N and Pb traces in the bulk.The presence of lead was also confirmed from the peak found at 20.6 eV (Figure S8, with fitting parameters in Table S5).S5: Spectral fitting parameters associated to the spectra in Figure S9.
The O 1s spectrum is dominated by a main peak at 530.5 eV, which corresponds to crystal bulk oxygens [5,6].Other components are detected at lower and higher energies in this energy interval.
In the O 1s spectrum of Transition Metal Oxides (TMOs), typically, two components can be discerned in the BE range 529.5-533.[7] The main peak is usually found in the BEs range 529.5-530.5 eV, which is ascribed to O 2-ions of the crystalline network.Moreover, a lateral structure is also present at higher binding energies (531-533 eV), and it is due to compensation of oxygen deficiencies in the subsurface.The main and lateral oxygen peaks of molybdenum oxide are usually found at 530.5 eV and 532 eV [6,7], respectively, which are in good agreement with our case.The deconvolution process of the O 1s spectrum reveals a further peak at 533.1 eV, which might indicate the existence of ionization associated with weakly adsorbed species.[7] However, the presence of this component can also be ascribed to organic contamination or adsorbed water molecules.[5,8] Moreover, a weaker component was disclosed at lower energies (529.0 eV), and in [5] it has been associated to defective Mo oxides.S11: Electrical parameters extrapolated from the J-V curves of Fig. 8-B in the main text.
The AVT of the ST-PSCs is also reported.

FigureFigure S5 :Figure
Figure S4: J-V characteristic of the gold-based opaque cell (black curve) and the semitransparent

Figure S7 :
Figure S7: 3D AFM height images and EDX elemental maps of vanadium oxide and molybdenum

2 Figure
Figure S8: XPS spectra of MoOx film in the energy range 390-420 eV.

Figure S12 :Figure S13 :
Figure S12: Normalized fill factor, short circuit current and open circuit voltage as a function of

2 )
Figure S14: Series resistance (RS) box plot as a function of the MoOx thickness, employed as PBL

Figure S19 :
Figure S19: Average Visible Transmittance (AVT, on the left) and perovskite thickness (on the

Table S1 :
Sheet resistance for each type of ITO film deposited on glass.The sheet resistance was measured with the four-point probe method.The measured value and the accuracy are reported.
Figure S2: Electrical parameters (JSC, VOC, FF) as a function of the set RF power density of the sputtering system.

Table S2 :
Oxygen and Argon concentrations are listed for each ITO sputter deposition that was tested.The sheet resistance was measured for each layer (glass/ITO) and is reported in the last column.FigureS3: Transmittance as a function of wavelength from 300 nm to 850 nm of glass/ITO.The layers differ for the set oxygen flow rate during the sputtering process.

Table S4 :
Spectral fitting parameters associated to the spectra in FigureS7.
Figure S9: XPS spectra of MoOx film in the BEs range 17-34 eV.

Table S6 :
Spectral fitting parameters of the XPS spectra in FigureS10.

Table S7 :
Spectral fitting parameters related to the fitting process of the V2Ox film.

Table S9 :
Mean and standard deviation of the electrical parameters of the ST-PSCs without PBL and with different thicknesses of PBLs.The parameters of the gold-based reference for the batch on V2Ox experiment and the batch on MoOx experiment are reported.The experiments were conducted on 10 samples for each category.

Table S10 :
On the left side, EQE, 1-R and integrated short circuit current as a function of wavelength of devices without PBL (ITO) and with PBL (V2Ox/ITO, MoOx/ITO).On the right side, maximum power point tracking over time of samples with the PBLs.Summarized electrical parameters extrapolated from the simulations with LTSpice.